Kinetic and mechanism investigation of the reactions between triphenylphosphine, dialkyl
acetylenedicarboxilates and NH-acids such
as 2-acetylpyrrole and 2-pyrrolecarbaldehyde by UV
Sayyed Mostafa Habibi Khorassani, Ali Ebrahimi, Mohammad Zakarianezhad and Puya Karimi
* Address Corresponding to Sayyed Mostafa Habibi Khorassani, Department of chemistry, The University of Sistan and Baluchestan, P.O.Box 98135-674, Zahedan, Iran; Tel: +985412446565;
Fax:+985412446565; E-mail address: [email protected]
Abstract
The works undertaken were to carry out kinetic studies of the reactions between triphenylphosphine, dialkyl acetylenedicarboxylates in the presence of NH-acid, such as 2-acetylpyrrole or 2-pyrrolecarbaldehyde. To determine the kinetic parameters of the reactions, they were monitored by UV spectrophotometery. The second order fits were automatically drawn and the values of the second order rate constant (k2) were calculated using standard equations within the program. At the temperature range studied the dependence of the second order rate constant (Ln k2) on reciprocal temperature was in agreement with Arrhenius equation. This provided the relevant plots to calculate the activation energy of all reactions. Furthermore, useful information were obtained from studies of the effect of solvent, structure of reactants (different alkyl groups within the dialkyl acetylenedicarboxylates and NH-acids) and also concentration of reactants on the rate of reactions. Proposed mechanism was confirmed according to the obtained results and steady state approximation and first step (k2) of reaction was recognized as a rate determining step on the basis of experimental data.
Keywords: Phosphorus ylide; NH-acid; UV spectrophotometry; Kinetic parameters;
Rate constant.
Introduction
Phosphorous ylides are reactive systems, which take part in many valuable reactions
of organic synthesis.1-5 These are most often obtained in excellent yields from the 1:1:1 addition reaction between triphenylphosphine, dialkyl acetylendicarboxylates, in the presence of strong CH, SH or NH-acids.6-10 A facile synthesis of the reaction between triphenylphosphine 1, dialkyl acetylendicarboxylates 2 and 2-acetylpyrrole 3a or 2- pyrrolecarbaldehyde 3b (as NH-acids) have been earlier reported10 but the kinetic studies of these reactions have not been investigated yet. In order to gain further insight into the reaction mechanism, a kinetic study of the reactions was undertaken by the UV spectrophotometer technique. Numerous kinetic investigations over a large area of different reactions have been previously reported using the UV instrument.11-14 On the basis of the well established chemistry of trivalent phosphorus nucleophiles, it is reasonable to assume that phosphorus ylide 4 results from the initial addition of triphenylphosphine to the acetylenic ester 2 (2c, 2d or 2e) and subsequent protonation of the 1:1 adduct by the NH-acids 3 (3a or 3b) to form phosphoranes 4 (4ac, 4ad, 4ae, 4bc, 4bd or 4be) (see Figures 1 and 2).
Figure 1
Figure 2
Methods
First it was necessary to find the appropriate wavelength in order to follow the kinetic study of the reaction. For this purpose, in the first experiment, 3×10-3M solution of compounds 1, 2e and 3a have been prepared in n-hexan as solvent. The reaction monitored by recording scans of the entire spectra every 12 min over the whole reaction time at the ambient temperature. The ultra-violet spectra shown in Figure 3 are typical. As can be seen from this Figure, the appropriate wavelength can be chosen 340 nm (corresponding mainly to the compound 4ae (product). In this wavelength, compounds 1, 2e and 3a have relatively no absorbance value.
This provided good opportunity in order to fully investigate the kinetic of the reaction between triphenylphosphine 1, di-tert-butyl acetylenedicarboxylate 2e and 2- acetylpyrrole 3a at 340 nm in the presence of n-hexan as solvent.
Figure 3
Conclusions
Kinetic investigation of the reactions between triphenylphosphin 1, dialkyl acetylenedicaroxylates 2 (2c, 2d or 2e), with 2-acetylpyrrole 3a or 2- pyrrolecarbaldehyde 3b were undertaken by the UV spectrophotometery. The results can be summarized as follow; 1. The appropriate wavelengths and concentrations were successfully determined to follow kinetic investigations. 2. The overall order of all six reactions followed second- order kinetic and order of reaction with respect to triphenylphosphine, dialkyl acetylenedicarboxylate and 2-acetylpyrrole or 2- pyrrolecarbaldehyde were obtained 1, 1 and zero respectively. 3. The values of second- order rate constant of all reactions were automatically calculated with respect to the standard equation within the software associated with Cary-300 UV equipment. 4. The rate of all reactions were increased in media of higher dielectric constant solvent, this can be related to differences in stabilization of the reactants and the activated complex
in transition state by solvent. 5. The more steric factor in bulky alkyl groups accompanied by its more inductive effect within the structure of dialkyl acetylenedicarboxylate would tend to reduce the rate of overall reactions. 6.
With respect to the experimental data first step of proposed mechanism was recognized as a rate determining step (k2) and reaction mechanism was confirmed based upon the obtained experimental results and also steady state approximation. 7. Reproductive experiments with different structure of NH- acids (3a and 3b) were indicated that they participate in the fast step of reactions.
Acknowledgments
Authors sincerely thank the University of Sistan & Baluchestan for providing financial support of this work.
Refrences
1. A.W. Johnson. Ylid Chemistry, Academic Press, New York (1966).
2. D.E.C. Corbridge, Phosphorus: An Outline of Chemistry, Biochemistry and Uses, 5th (Ed.), Elsevier, Amsterdam (1995).
3. K.M. Pietrusiewiz, M. Zablocka. Chem. Rev., 94, 1375 (1994).
4. K.C. Nicolaou, M.W. Harter, J.L. Gunzner, A. Nadin. Liebigs Ann., 1283 (1997).
5. Gilchrist, T. L. Heterocyclic Chemistry, Wily: NewYork, 1985.
6. I. Yavari, A. A. Esmaili, S. Asghari, H. R. Bijanzadeh. Chem. Res., 368 (1999).
7. I. Yavari, A. Alizadeh, M. Anary-Abbasinejad.Tetrahedron Letters.,177, (2002).
8. I. Yavari, M. Adib, F. Jahani-Moghaddam, M. H. Sayahi.
Phosphorus, Sulphur, and Silicon., 177, 545 (2002).
9. M. T. Maghsoodlou, N. Hazeri, S. M. Habibi khorassani, L.
Saghatforosh, M. K. Rofouei, M. Rexaie. Arkivoc., 117-123 (2006).
10. M. T. Maghsoodlou, N. Hazeri, S. M. Habibi Khorassani, M. Nassiri and etc all. Phosphorus, Sulphur and Silicon., 181, 865 (2006).
11. P. G. Furtmuller, U. Burner, G. Regelsberger. Biochemistry., 39, 15578-15584 (2000).
12. O. J. Nielsen, J. Sehsted, S. Langer, E. Ljungstrom, I.Wangberg.
Chem. Phys. Lett., 238, 359-364 (1995).
13. M. L. Ragains, B. J. Finlason-Pitts. J. Phys. Chem. A., 101, 1509-1517 (1997).
14. N. Marti, G. H. Hoa, B. Kozelka. J. Inorganic Chemistry Communications., 1, 439-442 (1998).
